Modern farmers aren't just planting potatoes. Besides, if you want to make a point that the MWP was warmer than today, the data needs to be calibrated to an actual temperature, and the data needs to be global.

It doesn't work within science. It works fine when trying to determine policy based on science. Everything we teach the kids at school is based on scientific consensus. That's why we teach the theory of evolution, even though there are some people that disagree. Even if there's no guarantee that scientific consensus is correct, it's still the best thing we have.

If 2-sigma events happen on a regular basis, they are unlikely to be a result of chance.

If you make a list of extreme weather events, the number of broken heat records is much bigger than the number of broken cold records.

I agree that people overreact, and start blaming each event on global warming. That's not correct. However, looking at the increase of dozens of extreme weather events, it's fair to say that global warming is a likely factor behind many of them.

they would drive most of the world into poverty and a brutish existence

The same thing will happen if we just continue to burn fossil fuels. We can't keep producing them at current rate for much longer. The peak oil problem is likely more urgent than global warming, so an aggressive plan for transition would benefit us either way.

we have no idea about what the real benefits/disadvantages we would experience from +450ppm.

Sure, we have plenty of ideas.

But I see your point. Short term benefits outweigh long term doubts. Since, long term, we're all dead anyway, I can't argue with that.